Two overriding questions unify all workers in the fibrinogen-fibrin field: how are the fibrin(ogen) units packed in the fibrin polymer, and what are the forces that hold them together? We are taking a twofold approach to these problems, one based on the evolutionary interdependence of protein structure and function, the other based on synthetic peptides patterned on the most conserved features of the system. At the same time we are naturally interested in how and when vertebrate blood coagulation in general was invented. Our proposal is based on the notion that only if we understand the history of a system can we intelligently manipulate it. Specific goals include further characterization of fibrinogen from the lamprey, the most primitive vertebrate extant and the most primitive creature known to have a thrombin-clottable fibrinogen molecule. We plan to complete the cDNA sequences of clones for the beta and alpha chains, as well as continue studies on peptide binding and fibrin polymerization. Knowledge gained from these studies will be applied in the two different realms. First, those features that mammalian and lamprey fibrinogens have in common must be vital to function, as evidenced by their having been conserved for over 400 million years. We plan to synthesize the most rigorously conserved parts of the fibrinogen molecule in order to identify specific sites for polymerization and other interactions. Second, features common to lamprey and mammalian fibrinogens are also those most likely to exist in even more primitive creatures, among whom fibrinogen has yet to be found. We intend to use the genetic equivalents of those features (i.e., appropriate restriction fragments and synthetic oligonucleotides) to screen genomic libraries prepared from protochordates and those invertebrates thought to be most closely related to the vertebrate line. The focus of the search will be on exon or domainal equivalents that may have been assembled from diverse sources in the construction of the portotype fibrinogen molecule. The study also encompasses a search for the antecedent genes of other plasma proteins usually thought to be exclusive to vertebrates, including albumin, transferrin, ceruloplasmin and alpha-2-macroglobulin.